1. Field of the Invention
The present invention relates to brightness control circuits, and more particularly, to an image brightness controlling apparatus and method, and an adaptive image brightness controlling apparatus and method based on the brightness degree and/or brightness range of an image, these apparatuses and methods being capable of maintaining the sense of color, such as the original hue (color tone) or chroma, while increasing the brightness of the entire image or part of the image displayed on an image display device, such as a display monitor or a color TV.
2. Description of the Related Art
An image brightness controlling apparatus is a circuit for controlling the brightness of a displayed video image. The image brightness control circuit disclosed in U.S. Pat. No. 4,717,953 applies a direct current (offset) to the red, green, and blue signals constituting an image signal. Accordingly, a conventional image brightness control circuit has a limit in increasing the brightness of an image.
A method of controlling the brightness of an image includes a method of controlling the brightness of an image by applying a gamma function to a luminance signal Y and a method of controlling the brightness of an image by applying a gamma function to each of the red, green, and blue signals (hereinafter, referred to as an RGB signal). In the former method, an RGB signal is divided into a luminance signal Y and two color-difference signals Cb and Cr or I and Q, the brightness of an image is increased by applying a gamma function to the luminance signal Y, and the resultant signals are converted back into the RGB signal.
However, this method may cause a problem in that colors that can be reproduced are out of the color gamut. This gamut problem means that a luminance signal Y with an increased brightness does not have two corresponding color-difference signals. In this case, an increased brightness image does not keep the original hue or/and chroma. This may cause degradation in the quality of image.
In the latter method, a nonlinear involution is applied to each component of an input RGB signal, and accordingly, an output RGB signal has a different ratio, that is, a different direction vector, from the input RGB signal, leading to color variation.
The brightness and contrast of an image can be increased by histogram equalization as presented by W. K. Pratt, in “Digital Image Processing”, Wiley, 1978, and R. C. Gonzalez and R. E. Woods, in “Digital Image Processing”, Addison-Wesley, 1993. However, in this histogram equalization, the vector direction of an output color may be changed from the vector direction of an input color, which leads to color variation.